Dwarf Fortress is famous among gamers, but I'm surprised it's getting noticed by muggles. Heh, heh. Seriously, this is a game for a niche market, but all game developers could learn something from it.
An excerpt from "Where Do Dwarf-Eating Carp Come From?":
Dwarf Fortress is barely a blip on the mainstream radar, but it’s an object of intense cult adoration. Its various versions have been downloaded in the neighborhood of a million times, although the number of players who have persisted past an initial attempt is doubtless much smaller. As with popular simulation games like the Sims series, in which players control households, or the Facebook fad FarmVille, where they tend crops, players in Dwarf Fortress are responsible for the cultivation and management of a virtual ecosystem — in this case, a colony of dwarves trying to build a thriving fortress in a randomly generated world. Unlike those games, though, Dwarf Fortress unfolds as a series of staggeringly elaborate challenges and devastating setbacks that lead, no matter how well one plays, to eventual ruin. The goal, in the game’s main mode, is to build as much and as imaginatively as possible before some calamity — stampeding elephants, famine, vampire dwarves — wipes you out for good.
Though its medieval milieu of besieged castles and mutant enemies may be familiar, Dwarf Fortress appeals mainly to a substratum of hard-core gamers. The game’s unofficial slogan, recited on message boards, is “Losing is fun!” Dwarf Fortress’s unique difficulty begins with its most striking feature: The way it looks. In an industry obsessed with pushing the frontiers of visual awe, Dwarf Fortress is a defiant throwback, its interface a dense tapestry of letters, numbers and crude glyphs you might have seen in a computer game around 1980. A normal person looks at ♠§dg and sees gibberish, but the Dwarf Fortress initiate sees a tense tableau: a dog leashed to a tree, about to be mauled by a goblin.
This bare-bones aesthetic allows Tarn to focus resources not on graphics but on mechanics, which he values much more. Many simulation games offer players a bag of building blocks, but few dangle a bag as deep, or blocks as small and intricately interlocking, as Dwarf Fortress. Beneath the game’s rudimentary facade is a dizzying array of moving parts, algorithms that model everything from dwarves’ personalities (some are depressive; many appreciate art) to the climate and economic patterns of the simulated world. The story of a fortress’s rise and fall isn’t scripted beforehand — in most games narratives progress along an essentially set path — but, rather, generated on the fly by a multitude of variables. The brothers themselves are often startled by what their game spits out. “We didn’t know that carp were going to eat dwarves,” Zach says. “But we’d written them as carnivorous and roughly the same size as dwarves, so that just happened, and it was great.”
Note that there are graphics packs available, so you're not completely restricted to ASCII code. I'm not quite that hardcore, myself. But it's still very much a niche product. I love it, but that opinion won't be universal.
If you do want to try it - and why not? it's free - I recommend using the Lazy Newb Pack. I'd skip the third-party utilities, at least at first, but definitely use a graphics pack.
I won't quote any more of this article, since it's several months old already. If you want to read more, here's the link. (Registration is required, though it's free.) It's quite an interesting article.